In recent years, miniaturization of elements has been advanced along with the development of integrated circuits and various high-performance elements. Accordingly, thinning of a gate insulating film of a transistor used as a switching element or the like has been considered.
However, when the gate insulating film is thinned, defects may occur such as leakage current flowing between a gate electrode and a semiconductor film or a semiconductor substrate, which lowers the reliability of a semiconductor device. Therefore, an insulating film with a high withstand voltage has been demanded.
For example, Patent Document 1 discloses a technique to increase the withstand voltage of an oxide film by implanting oxygen ions at the time of forming the oxide film and then by performing heat treatment thereon.
Moreover, it is known that in a general process of forming an oxide film, the oxide film easily takes in hydrogen. By the hydrogen taken into the oxide film, O—H bonds are easily generated, which are the cause of deterioration since the O—H bonds in the oxide film become electron traps to lower the withstand voltage of the oxide film, make a threshold value of an MIS (Metal Insulator Semiconductor) type electric field effect transistor (FET: Field Effect Transistor) vary, and so on. Therefore, the gate insulating film needs to be a film with few electron traps.    [Patent Document 1] Japanese Published Patent Application H5-55200